Demand for special papers has been increasing for many years. This is particularly true for water resistant papers intended for the manufacture of abrasive papers, adhesive papers, artificial leathers, construction and coating papers and boards, insulation, packing, etc. It is principally polymeric materials which confer on these special papers their impermeability, their flexibility and/or their resistance, and it is principally phenolic resins (phenoplasts) and amino resins (aminoplasts) which heretofore have permitted different appropriate coatings to be laid thereon which the manufacturers of these different special papers have required.
Thus, while the literature is rich with its disclosure of various methods for incorporating latex in the fibrous mixture fed into a paper machine, and such proceses have been tested extensively for more than twenty years, the results obtained have never been satisfactory from the exploitation point of view and very many economic and technological difficulties remain unresolved, particularly when the proportion of latex attempted to be incorporated exceeds 7-10% of the fibrous mass. It is for these reasons that the technique of post-impregnation of the already fabricated paper sheet is currently and very widely used.
Accordingly, to produce these different special papers, the conventionally used procedure still remains the following: (1) A continuous sheet of paper is formed on a paper-making machine and is wound on a mandrel so as to form a reel; (2) the reel so obtained is then fed in a continuous length through another machine in one, two, three, or even more, passes for the purpose of depositing on one or both faces of the fibrous sheet materials different layers such as latex, resins, waxes, tars, etc. It is only then that the sheet, after appropriate drying, is again wound in a reel. Thus, in order to obtain the desired paper, it is necessary to repeat the winding and unwinding operations a plurality of times, applying each time a new coating. That obviously considerably increases the price of the product. Moreover, it is difficult to maintain porosity as well as the control of other physical properties, through the impregnation technique, and in addition undue quantities of impregnating media are used.
Returning to the many attempts over the past twenty years or more to make impregnated paper by the use of latex during the initial formation of the paper web, it has previously been the objective of workers in the field to cause the coagulation of the latex and its precipitation on the paper-making fibers. Unfortunately, when the latex coagulates it also deposits as beads on the paper-making wire and on the various tanks and other equipment, which rapidly makes further processing difficult if not impossible. This problem may also be due in part to the generally poor retention of the latex and other resins which may be used. Nevertheless, despite the expectation of a spectacular lowering of the cost if the latex or other additives could be added into the body of the pulp in the paper-making machine, this simplification of manufacture has not yet been successfully implemented commercially when it is desired to utilize more than about 7-10% of latex based on the fibrous mass, and accordingly recourse is almost invariably had to the difficult and costly process of post-impregnation. It is for these reasons that the technique of post-impregnation of the already fabricated sheet is currently and very widely used.